In recent years, under social demands for energy conservation, in order to improve fuel consumption of a vehicle, durability of a tire has been required, and thus a variety of rubber materials with excellent fracture resistance, abrasion resistance, and crack-growth resistance have been desired. Also, due to soaring prices of natural rubber, it has been required to develop synthetic rubber having the durability similar to that of the natural rubber. Although synthetic polyisoprene has conventionally been used as an alternative to the natural rubber, there has been a problem that the synthetic polyisoprene used as a rubber composition has poor durability as compared with the natural rubber. Therefore, in order to improve the durability of the synthetic polyisoprene, it has conventionally been performed to improve elongation crystallinity by structuring synthetic polyisoprene having higher cis (For example, see Patent Documents 1 and 2).
There have also been known techniques that improve a desired rubber property by kneading the synthetic polyisoprene together with other polymer components, or by using a copolymer composed of isoprene and other monomers. For example, Patent Document 3 discloses a rubber composition for a tread of a tire composed of a styrene-isoprene copolymer and a styrene-butadiene copolymer. Also, Patent Document 4 discloses a rubber composition containing butadiene-isoprene as a rubber component.